1. Field of the Invention
This invention relates, in general, to electrostatic powder coating and, more specifically, to the construction of an electrostatic powder coating installation.
2. Description of the Prior Art
One of the more important processes for applying coating materials to commerical products is the use of electrostatic powder coating techniques. Briefly, this technique involves the application, such as by spraying, of powder particles, which have a predetermined electrical charge applied thereon, to a workpiece which is at ground potential. The charged powder particles are attracted to the workpiece and form a uniform coating therearound which is subsequently fused by the application of heat to form a solid homogeneous coating.
In the electrostatic powder coating process, relatively large portions of the released electrostatically charged powder do not impinge upon the workpiece or do not have sufficient adherence thereto, for instance, because the desired layer thickness has already been attained on the workpiece. This portion, or excess powder, deposits like dust on the parts of the installation or spray booth. This dust must be removed to prevent undesirable buildup or for possible reapplication to the workpiece.
A state-of-the-art installation in which the excess powder or overspray is continuously collected is illustrated in U.S. Pat. No. 3,918,641. The electrostatic powder coating installation depicted therein consists of an enclosure in which there are arranged means, coupled with a spray container, for the purpose of delivering electrostatically charged powder. A part of the interior of the enclosure is bounded by one face of a filter belt moving through the enclosure; the other face of which is in confronting relationship with the suction side of a suction device which is effective through the filter at the aforementioned interior of the enclosure. A cleaning device is associated in operative relationship with the filter belt for removing therefrom excess powder which has been deposited on one face thereof.
In this installation, the greatest portion of the excess powder falls onto the filter belt under the action of the suction device so that the remaining parts of the installation are considerably less contaminated by deposits of the powder. By means of the cleaning device, the excess powder is removed from the filter belt and delivered to a collecting or supply container. The cleaning device is equipped with a suction nozzle directed towards one face of the filter belt. The suction nozzle is connected through a separator to the suction side of a blower with the outlet of the separator connected to the supply container. In this manner, the excess powder and exhaust air within the enclosure are drawn, under the influence of the suction blower, to the filter belt which has a predetermined permeability such that the air is purified of the excess powder as it is drawn through the filter belt and withdrawn from the enclosure; while the powder is collected on the surface of the filter belt and recovered by the cleaning device for reapplication to the workpiece.
Although such an installation provides economical operation insofar as removing and collecting the excess powder from the enclosure and reapplying the powder to the workpiece in a continuous operation, considerable difficulties have been encountered in attaining acceptable reliability or useful life of the filter belt. The downward pressure exerted by the suction blower on the powder and air within the enclosure causes the filter belt to be forced into engagement with the top surface of an exhaust plenum, which is typically covered by wire screen or grate. The sliding action of the filter belt over the wire screen as it moves through the enclosure causes rapid abrasion of the filter belt necessitating replacement of the filter belt after a relatively short interval of use. Not only does the replacement of the filter belt require considerable down time of the powder coating installation, but also additional expense is incurred in order to refurbish the worn filter belt or to purchase additional belts which decreases the economical advantages afforded by powder coating techinques over other types of coating methods.
Thus, it is desirable to provide an electrostatic powder coating installation having a movable filter belt for continuously collecting excess powder within the enclosure in which abrasion of the filter belt caused by the sliding contact between the filter belt and the exhaust plenum, as the belt moves through the enclosure, is minimized. It is also desirable to provide the aforementioned electrostatic powder coating installation in which abrasion of the filter belt is minimized and, at the same time, the filtering characteristics of the filter belt remain unchanged.